Four murine embryo cell lines have been tested for mitogenic response to a series of proposed mitogens (insulin, FGF, EGF, PMA, PGF2 alpha). Each cell line exhibits a separate and unique response spectrum. Independent cell lines are being selected which lack proliferative responses to specific mitogens. These lines are then being tested for proliferative responses to other agents and also for receptor binding capacity. Variants which are "conditionally non-proliferative" (i.e. temperature sensitive) are also being selected.